“Isomerase” is a generic term meaning a enzyme catalyzing conversion of isomers. According to “Enzyme Nomenclature”, Academic Press Inc., USA, 1992, it includes the following six groups: (1) EC 5.1; racemases and epimerases catalyzing optical isomerization, (2) EC 5.2; enzymes catalyzing geometric conversion of cis-trans isomers, (3) EC 5.3; enzymes catalyzing aldose-ketose conversion, keto-enol tautomerization, and intramolecular rearrangement of double bond, (4) EC 5.4; enzymes catalyzing intramolecular rearrangement of substituent to produce structural isomers, (5) EC 5.5; enzymes catalyzing intramolecular lyase-reaction, and (6) EC 5.99; enzymes catalyzing other isomerization. Among these isomerases, for example, the following enzymes are well known as isomerases catalyzing isomerization of neutral saccharides: xylose isomerase (EC 5.3.1.5) catalyzing conversion between D-xylose and D-xylulose, or between D-glucose and D-fructose (aldose-ketose conversion), aldose 1-epimerase (EC 5.1.3.3) catalyzing conversion between α and β anomer of aldose, ketose 3-epimerase catalyzing epimerization of C-3 position of ketopentoses and ketohexoses to produce the corresponding epimers (q.v. Japanese Patent Kokai No. 125776/1994 or International Patent Publication No. WO 2007/058086). These enzymes are widely used for industrial production of isomerized saccharides, quantitative determination of saccharides, and preparation of rare saccharides.
On the other hand, Tyler et al., Archives of Biochemistry and Biophysics, Vol. 119, pp. 363-367 (1967), reported that Ruminococcus albus, an anaerobic bacteria, produces a cellobiose 2-epimerase, and it epimerizes C-2 position of reducing-terminal glucose in cellobiose to produce epicellobiose (4-O-β-D-Glucosyl D-mannose), which enzyme has been assigned a enzyme number of EC 5.1.3.11 in Enzyme Nomenclature referred to above. Ito et al., Biochemical and Biophysical Research Communication, Vol. 360, pp. 640-645 (2007) and Ito et al., Applied Microbiology and Biotechnology, Vol. 79, pp. 433-441 (2008) disclosed the amino-acid sequence of the cellobiose 2-epimerase, the DNA sequence encoding the amino-acid sequence, and that the cellobiose 2-epimerase acts on cellooligosaccharide or lactose, as well as cellobiose, to produce epicellooligosaccharide or epilactose (4-O-β-D-calactosyl D-mannnose). Furthermore, Taguchi et al., FEMS Microbiology Letters, Vol. 287, pp. 34-40 (2008), disclosed cellobiose 2-epimerase produced by Eubacterium cellulosolvens, also an anaerobic bacteria.
Nishimukai et al., Journal of Agricultural and Food Chemistry, Vol. 56, pp. 10340-10345 (2008) disclosed that when epilactose, converted from lactose by cellobiose 2-epimerase, was ingested in rat, it exerted physiological functions of promoting calcium absorption in the small intestine, increasing the amount of short-chain fatty acid in the intestine, and lowering the plasma cholesterol level, suggesting that epilactose is expected to be developed for a prebiotic material.
However, the above known cellobiose 2-epimerase have problems that they are hard to use for industrial production of epilactose or epicellobiose because of their low heat-resistance. Heat-resistance is an important property for practical application of enzyme reaction, and a highly heat-resistant enzyme is economically beneficial because a longtime reaction can be carried out with a small amount of the enzyme, resulting in low consumption of the enzyme. In consideration of industrial use, enzyme reaction is preferable to be conducted at 55° C. or more, preferably, 60° C. or more. In the above regard, a cellobiose 2-epimerase with higher heat-resistance is desired.